Self-boosting circuit for a power amplifier

ABSTRACT

A power amplifier circuit includes an amplifying transistor and a dc bias circuit for biasing the amplifier transistor to obtain a conduction angle of at least about 180°. The dc bias circuit includes a self-bias boosting circuit for initially decreasing and then increasing the dc bias voltage provided to a control terminal of the amplifying transistor by the dc bias circuit as the input signal provided to the power amplifier increases. The self-bias boosting circuit is extremely simple and compact in design, and permits the power amplifier circuits to operate in Class B or Class AB with improved power output characteristics.

BACKGROUND OF THE INVENTION

[0001] The invention is in the field of transistor amplifier circuits,and relates more particularly to a power amplifier circuit having aself-bias boosting circuit for increasing maximum power output andreducing power dissipation at low power levels.

[0002] Amplifiers of this general type are frequently used inhigh-frequency RF amplifiers, as well as in audio amplifiers and otherapplications. In order to obtain a linear input-output relationship andhigh operating efficiency, such amplifiers are typically operated with aconduction angle of about 180° (Class B) or slightly greater (Class AB)to avoid crossover distortion.

[0003] Typically, amplifiers of this type require a dc bias circuit toestablish the quiescent bias current in the amplifier circuit to ensureoperation in the Class B or Class AB mode. In the prior art, bias istypically provided by a fixed current source, as shown in U.S. Pat. No.5,844,443, or else by an external supply, which can be set to a desiredconstant value to secure the quiescent current necessary to operate inthe desired mode, as shown in U.S. Pat. No. 5,548,248.

[0004] However, in amplifiers of the type described above the averagecurrent drawn from the supply depends upon the input signal level. Asthe output power increases so does the average current in both theemitter and the base of the power transistor. This increased averagecurrent causes an increased voltage drop in the biasing circuitry and inballast resistors (which are used to avoid hot-spotting and thermalrunaway in transistors using an interdigitated design). This in turnreduces the conduction angle (i.e. the number of degrees out of 360°that the amplifier is conducting), and forces the amplifier deep intoClass B or even Class C operation, thereby reducing the maximum poweroutput. To avoid this power reduction, the amplifier must have a largerquiescent bias. In prior-art circuitry this inevitably leads to a higherpower dissipation at low power output levels and therefore anundesirable tradeoff in operating characteristics.

[0005] A recent improvement in this art is disclosed in co-pending U.S.patent application Ser. No. 09/536,946, entitled Dynamic Bias BoostingCircuit For A Power Amplifier, filed on Mar. 28, 2000 by Sowlati andLuo, two of the present inventors. This application discloses a solutionto the problem discussed above which entails providing the poweramplifier circuit with a dynamic bias boosting circuit to dynamicallyincrease the bias of the power transistor as the output power increasesby using a circuit that senses the input voltage to the amplifier andgenerates a dynamic bias boost as a function of the amplitude of thissignal. The drawback to this solution is that it employs numerous activeand passive components, thus not maximizing simplicity, compactness andeconomy of manufacture.

[0006] Accordingly, it would be desirable to have a power amplifiercircuit which offers the advantages of optimum maximum output power andreduced power dissipation at low power levels. Additionally, it would bedesirable for such a circuit to be extremely simple and compact indesign, and very economical to manufacture.

SUMMARY OF THE INVENTION

[0007] It is therefore an object of the present invention to provide apower amplifier circuit which provides improved maximum output power andless power dissipation at low power levels. It is a further object ofthe invention to provide a power amplifier circuit which is bothextremely simple and compact in design and which is very economical tomanufacture.

[0008] In accordance with the invention, these objects are achieved by anew power amplifier circuit for amplifying an input signal and having aconduction angle of at least about 180°, the amplifier circuit includingan amplifying transistor and a dc bias circuit for biasing the amplifiertransistor to obtain the desired conduction angle. The dc bias circuitincludes a self-bias boosting circuit for initially decreasing and thenincreasing the dc bias voltage provided to a control terminal of theamplifying transistor by the dc bias circuit as the input signalprovided to the power amplifier increases.

[0009] In a preferred embodiment of the invention, the amplifier circuitis either a Class B or a Class AB amplifier circuit.

[0010] In a further preferred embodiment of the invention, the self-biasboosting circuit includes a capacitor coupled from an output terminal ofthe self-bias boosting circuit to a common terminal, and a resistorcoupled between the output terminal and the control terminal of theamplifying transistor.

[0011] In yet a further preferred embodiment of the invention, theself-bias boosting circuit also includes a switch coupled in series withthe capacitor for enabling the power amplifier circuit to operate ineither of two output power modes.

[0012] A power amplifier circuit in accordance with the presentinvention offers a significant improvement in that a particularlyadvantageous combination of features, including increased maximum outputpower and reduced power dissipation at low power levels, can be obtainedin an extremely simple, compact and economical configuration.

[0013] These and other aspects of the invention will be apparent fromand elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWING

[0014] The invention may be more completely understood with reference tothe following description, to be read in conjunction with theaccompanying drawing, in which:

[0015]FIG. 1 shows a simplified schematic diagram of a power amplifiercircuit in accordance with a first embodiment of the invention; and

[0016]FIG. 2 shows a simplified schematic diagram of a power amplifiercircuit in accordance with a second embodiment of the invention.

[0017] In the drawing, like reference numerals are generally used todesignate like components.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] A simplified schematic diagram of a high-frequency poweramplifier circuit 1 is shown in FIG. 1 of the drawing. The amplifiercircuit includes an amplifying transistor 10 and a bias circuit 2coupled to the base of the amplifying transistor 10 by a resistor 11.The bias circuit 2 includes bipolar transistors 12 and 13, coupled inseries between V_(cc) and a common terminal (GND), with the transistorsbeing coupled to bias voltage supplies shown in simplified form by biasvoltage supplies 14 and 15 coupled to the bases of transistors 12 and13, respectively, by resistors 16 and 17. The basic circuitconfiguration is completed by an input coupling capacitor 18 forcoupling an input signal Vin to the base of amplifying transistor 10,with the transistor 10 being connected in a common-emitter configurationand coupled between V_(cc) and GND by an inductor 19. The output ofpower amplifier circuit 1 is taken from the collector of transistor 10and is designated as V_(out) in the drawing.

[0019] In connection with the circuit as so far described, it should beunderstood that although the active components are shown as bipolartransistors for illustrative purposes, field effect transistors or acombination of bipolar and field effect transistors may alternatively beused within the scope of the invention. Additionally, it is to beunderstood that the power amplifier circuit 1 and bias circuit 2 maydiffer in form and detail from the simplified, illustrative depictionsshown in the drawing. Furthermore, it is to be understood that the biassupply may be configured and adjusted to permit the amplifier circuit tooperate in either Class B or Class AB mode.

[0020] In accordance with the invention, capacitor 20 is coupled from anoutput terminal 21 of the bias circuit 2 to a common terminal (here GND)in order to achieve a self-bias boost feature in an extremely simple,compact and economical manner. In an illustrative but nonlimitingexample, in a circuit suitable for use at wireless telephonecommunication frequencies, capacitor 20 may have a value of about 2.2 pFand resistor 11 may have a value of about 15 ohms. Additionally, in apractical implementation, the bias voltages supplied to transistors 12and 13 by components 14, 15, 16 and 17 could be supplied by other biascircuits already existing in other portions of a device incorporatingthe power amplifier circuit of the invention, thus further simplifyingthe circuit and reducing cost and size.

[0021] As opposed to more complex and costly prior-art bias controlcircuits, the present invention provides an extremely simple andautomatic improvement in circuit performance by using capacitor 20together with the other circuit components, with the capacitor acting asa self-bias booster in the amplifier circuit, in a manner which will beexplained in further detail below.

[0022] A further preferred embodiment of the invention is shown in thesimplified schematic diagram of FIG. 2. The circuit shown in FIG. 2incorporates all of the components of FIG. 1, with like referencenumerals designating like components, and accordingly these componentswill not be described further here. The circuit of FIG. 2 differs fromFIG. 1 in that it additionally includes a switch 22 coupled in serieswith capacitor 20, so that the two series-connected components areconnected between terminal 21 and GND. In this manner, with the additionof only a single additional component, the power amplifier circuit canoperate in either of two different output power modes, namely a higherpower mode (in which capacitor is connected to ground) and a loweroutput power mode (in which switch 22 is opened to disconnectedcapacitor 20 from the circuit). When the circuit is used in wirelesscommunication applications, for example, this dual power mode embodimentmay be employed to switch power levels in order to enable operation inboth analog and digital modes, which typically employ different powerlevels. Although circuit element 22 is shown in FIG. 2 as a simpleswitch, it will be understood that this component may be implemented inpractice by either a mechanical switch or a semiconductor device such asa transistor or a PIN diode.

[0023] In the foregoing circuits, the capacitor 20 acts as a self-biasbooster in the amplifier circuit. This feature, when used with properinput and output matching networks, will provide improved maximum outputpower and less power dissipation at low power levels.

[0024] In order to better understand the operation of the instantinvention, consider the operation of the circuit of FIG. 2 with switch22 opened so that capacitor 20 is effectively disconnected. In thismode, the average voltage drop across resistor 11 will increase as theRF input level (Vin) increases. This will in turn cause a reduction inthe base-emitter voltage of transistor 10, thus pushing this transistorinto saturation. When switch 22 is closed, thus connecting capacitor 20into the circuit, the base-emitter voltage of transistor 10 willinitially decrease and then subsequently increase as the input signalprovided to the power amplifier circuit increases, thus helpingtransistor 10 to stay within the desired operating region and permittingimproved maximum output power within the desired mode of operation.

[0025] It is to be emphasized that although the improvement in theinvention is extremely simple, compact and economical to implement, itis based upon a recognition of certain operating conditions that are notapparent or obvious. More particularly, the invention is based upon arecognition that, at lower input power levels, the discharging rate ofcapacitor 20 through transistor 12 is equal to or faster than itscharging rate through transistors 10 and 13. At higher power levels, onthe contrary, the discharging rate of capacitor 20 becomes slower thanits charging rate. Accordingly, as the input signal provided to thepower amplifier circuit is increased, the DC bias voltage provided tothe base of transistor 10 will initially decrease and then increase,thus providing the desired self-bias boosting effect in an extremelysimple and economical manner. Computer simulations indicate that theprovision of this single component (capacitor 20) can improve themaximum output power available by up to about 40%.

[0026] While the invention has been particularly shown and describedwith reference to several preferred embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetail, some of which have been suggested above, may be made withoutdeparting from the spirit or scope of the invention. Thus, for example,different types of transistors may be employed, and alterations to thecircuit configuration may be made to suit particular designrequirements.

What is claimed is:
 1. A power amplifier circuit for amplifying an inputsignal and having a conduction angle of at least about 180°, said poweramplifier circuit comprising an amplifying transistor and a dc biascircuit for biasing said amplifying transistor to obtain said conductionangle, and said dc bias circuit comprising a self-bias boosting circuitfor initially decreasing and then increasing a dc bias voltage providedto a control terminal of said amplifying transistor by said dc biascircuit as said input signal provided to said power amplifier circuitincreases.
 2. A power amplifier circuit as in claim 1, wherein saidamplifier circuit is a Class B amplifier circuit.
 3. A power amplifiercircuit as in claim 1, wherein said amplifier circuit is a Class ABamplifier circuit.
 4. A power amplifier circuit as in claim 1, whereinsaid self-bias boosting circuit comprises a capacitor coupled from anoutput terminal of said self-bias boosting circuit to a common terminal,and a resistor coupled between said output terminal and said controlterminal of the amplifying transistor.